If logging in via the guest account works fine, but you are having problems when logging into your account, it must be something to do with your account.
Either some personal settings, or some other programs/processes that are auto-starting.
I seem to remember from a previous post, that you said that this was happening when you changed the theme of the desktop. Changing the theme shouldn't really make much difference to the login time.
Have you got any large programs or processes that auto start when you log in? Or do you have a lot of widgets opening on the desktop? That can affect the time it takes for the desktop to load after log-in.
Also, when you log into KDE/Plasma using your account, is it set to restore a previous Plasma session, or is it set to start a completely new session?
If it is restoring a previous session rather than starting a new session, that could account for some slowness too. Because the desktop will be loading whatever applications were running the last time you logged out of plasma and attempting to restore the state they were in from data that is saved to disk....
After logging into a KDE/Plasma session and waiting for the desktop to load, I think the first thing you should do is open the system settings. Take a look at the 'session' settings in the 'startup and shutdown' category and ensure that you are starting a new session when you log-in.
Also, take a look in the 'service manager' settings in 'startup and shutdown' and see if there are any unneeded programs/services that are started when you log-in. Disable any that you do not need. e.g. On my laptop, I don't have any bluetooth hardware, so I've disabled the bluetooth related drivers/services (they used to auto-start by default in KDE).
Also try turning off the advanced desktop effects (just temporarily, I know you like your wobbly windows etc...) and remove any widgets that are on the desktop. If anything, it's most likely to be some heavyweight widgets on your desktop slowing things down. I don't think the desktop effects will have an impact on the desktops load time at login, but it couldn't hurt to try disabling them temporarily too to see what effect it has.
After all of that, try logging out and then log back in again. With any luck you should get to the desktop a little quicker!
If that doesn't improve things: As a last resort, you could try logging into an Enlightenment desktop session and then either remove or rename the .kde folder in your home folder. e.g. mv ~/.kde/ ~/oldKDESettings/
Then if you log out and log back into Plasma the system should recreate the .kde folder with default settings. This might take a little time, but if you log out and back in yet again, you should see some speed-up.
Then you will have to go back to the system settings and double check your session settings and the startup program settings etc. Then you can incrementally restore your preferred theme. Log out and back in again periodically to see if any of your changes have caused the login to slow down. It will be a pain in the backside logging in and out all the time, but it should allow you to pinpoint exactly which changes are causing login to slow down.
Once you know which things are causing the slow-down, you can simply avoid using them!
From personal experience, it is most likely to be session settings, or widgets on the desktop. Especially if they are non-plasma widgets, or if it is one of the more heavyweight plasma widgets!
Other than that, I'm pretty much out of ideas!